Burglar alarm system



Sept. 6, 1938. H. CHASE ET AL 2,129,261

BURGLAR ALARM SYSTEM Filed May 2, 1936 ckYsmL SUBSCRIBER U 7 kw ATTORNEY Patented Sept. 6, 1938 PATENT OFFICE 2,129,261 BUBGLAR ALARM SYSTEM Leland H. Chase, Little Neck, and Clifford L. London, New York, N. Y., assignors to Holmes Electric Protective Company,

a corporation of New New York, N. Y., York Application May 2, 1936, Serial No. 77,522

4 Claims.

This invention relates to burglar alarm systems and more particularly to burglar alarm systems which are maintained and controlled from a central alike.

The object of this invention is to provide a burglar alarm system which will register an alarm at the central ofilce when an opening is made in the walls of a protected area either by opening a door or window or cutting a hole through the wall to gain entrance to the vault or protected room.

Another object of the invention is the provision of means controlled from the central oflice for simulating an opening in the protected area for testing purposes.

Heretofore burglar alarm systems have been constructed so that an alarm is transmitted to the central ofiice when electric contacts on doors or windows are opened or closed; and when wires on walls, doors or windows are broken or brought into contact with each other; also various systems have been devised which use photoelectric cells in combination with light beams, the interruption of which closes a circuit to cause an alarm to be sounded.

While the above systems are satisfactory, they do have some drawbacks, are costly to install and need considerable supervision.

The present invention provides a burglar alarm system which may be installed and maintained with a minimum amount of expense and which may be supervised with a minimum amount of eifort.

In applicants system the air in the protected enclosure is maintained at a pressure difierent from that in an adjoining area by means of an electric fan or other type of electrically operated air pump, which is mounted in an aperture in the wall of the enclosure. It is not important that the enclosure be air-tight as long as the leakage is constant. This diiference in pressure is transmitted from the enclosure to the adjoining area or vice versa through a hollow tube to a metallic bellows. The difference in pressure on the inner and outer surface of the bellows wall will cause it to assume an abnormal length. One end of the bellows is arranged to act as one of the elements of an electric condenser in conjunction with an adjacent fixed parallel plate. This condenser is connected across the tuned plate circuit of a high frequency oscillator whose grid circuit has an extremely high Q characteristic afforded by a piezoelectric quartz crystal. The plate circuit of the oscillator is tuned by a variable condenser so that the circuit has a slightly lower natural frequency than that of the crystal excitation; that is, the plate circuit is near resonance with the crystal but has a capacity reactance. resulting in an impedance considerably lower than that at resonance and with a very steep plate current characteristic relative to slight tuning capacity changes in either direction.

In operation, if the value of pressure difference between the enclosure and the adjoining area be changed, either by changing the speed of the fan, or by making an opening in the enclosure wall, the length of the bellows will change. Thus the distance between the fixed plate and the bellows is varied, their action changing the 1 impedance of the high frequency oscillator plate circuit with a resultant change in the oscillator plate current. By means of a series of relays the change in the oscillator plate current, whether positive or negative, is transmitted to the central ofiice over a pair of wires where a zero center galvanometer and additional relays operate visual and audible alarms. This same apparatusconstitutes an alarm indicating device for any failure-of local power or apparatus. Additional equipment at the local and central office locations provides means for testing the switching and operation of the entire apparatus.

A signaling key and an alternating current bell at the subscribers location, and a switching key provided with a source of alternating current at the central office, provide a means for exchange of signals between the subscriber and the central oifice operator to afford cooperation at opening and closing times.

Referring now to the drawing, the enclosure ID has mounted in the wall thereof an electrically driven fan H which will either draw air from the enclosure Ill or pump it in. In either case the action of the fan II will cause a change in the air pressure between the'interior of the enclosure l0 and the adjoining area which may be in the order of .1 to .3 inch of water level. A hollow tube l2 transfers this pressure condition from the interior of the enclosure to the interior 1 of a flexible metallic bellows l3. Due to the difference in pressure inside and outside of the bellows I3, it will either expand or contract along its axis, depending upon whether or not the pressure on the inside is greater or less, respectively than the pressure on the outside. The T connection I at the junction of the tube l2 and bellows l3, afiords a point whereby the pressure difference may be measured by means of a suitable pressure gauge (not shown). During the normally closed pressure diflerence change.

period. that is, when the enclosure in is protected, and all the openings into the enclosure are closed the plate circuit of the oscillator I5 is tuned by means oi the condenser IS. The plate circuit is so tuned that the capacity in the circuit is slightly greater than the amount necessary for resonance with the piezoelectric crystal I! in the grid circuit. The exact setting is determined by the plate current characteristic of the oscillator as shown and described in C. L. London Case 1, Serial No. 693,252, filed October 12, 1933.

Inasmuch as the capacity between the fixed plate II and the metallic bellows is connected in parallel across the tuned plate circuit through condensers and 2|, it is a part oi! the tuned circuit, and any change in its value will cause a change in the plate current 01' the oscillator II. In making the initial adjustments the distance between the fixed plate and bellows is adjusted to give the desired plate current change for any justment is followed by a re-adjustment oi! condenser It to restore the proper electrical capacity condition.

The overload relay 22 is so adjusted that its contacts will close when the current through its winding increases to 8 milliamperes or more, and the underload relay 23 is so adjusted that its contacts remain closed as long as the current through its winding is 3.5 milliamperes or more, but willopen when the current falls below 3.5 milliamperes.

The resistances 2i and are so adjusted that the direct current from the central ofilce. through the alternating current bell 26, the changer magnet 21, the differential relay 28, the resistances 2| and 25 and the galvanometer 29 is 12 miniamperes and will rise to 15 milliamperes when the resistance 24 is short-circuited.

The galvanometer 29 is so adjusted that at 12 milliamperes the needle will be in the center of the scale and at 15 milliamperes it will be halfway between the center and the right-hand end of the scale. The resistance is adjusted to such a value that when the short circuit is removed therefrom the speed of the fan II will be reduced by an amount which will give the same pressure diflerence change as the minimum sized hole in the enclosure III, which it is desired to protect; that is, if an alarm is required from an opening of 16 square inches the Ian speed must reduce to give the same pressure difference change as an opening of that area would give.

Under the above conditions a pressure change inside the enclosure l9 will cause the bellows ii to increase or decrease in length depending on the direction of the pressure change. For example, when the bellows l3 increases in elongation, the electrical capacity of the tuned plate circuit will be increased due to the decrease in distance between the end of the bellows i3 and the hired plate l8. This results in a lowering of the impedance of the plate load of the tube 3|, which causes a decrease in the high frequency voltage fed back to the grid of the tube 3! through inter-electrode capacities. This action in turn causes a decrease in the negative bias on the tube 3| due to a diminishing of rectified grid current through the grid leak 32 and will cause an increase in the plate current of the oscillator.

When the displacement of the bellows l3 has progressed far enough to increase the oscillator plate current to 8 milliamperes the contacts of the relay 22 will close, short-circuiting resistance Any fixed plate ad- 24 and winding 2! 01' the differential relay 2.. The current through its other winding 2| being unopposed by the winding 28 will close the relay contacts, short-circuiting the resistance 2|, thereby causing a large increase of current to flow through the galvanometer 29, which causes contact 29- to close, and energize relay 32 which through its contacts will light the lamp 34 as a visual signal, and the lamp current through the relay 2! will operate the audible signal 38.

The above condition is maintained by the armature and contact 01 the diflerential relay 2| providing a locking circuit which maintains the short circuit on the resistances 24 and 25 and on the winding 28, even though the pressure difference, bellows I3, the plate current of the oscillator l5, and the relay 22 return to their original state.

In the condition where a change in the pressure difference between the interior oi the enclosure lll and the adjacent area causes the bellows I! to contract along its axis, the efifective capacity in the tuned plate circuit of the oscillator I! will become less, thereby causing the capacity in excess of the grid circuit resonance value to decrease. This will increase the plate circuit impedance, the voltage transferred to the grid and consequently the negative bias on the grid. The net result will be a decrease in the oscillator plate current. When this plate current has dropped below 3.5 milliamperes the contacts of relay 23 will open, opening the line circuit through the galvanometer 29 and causing the contact 29 to close, operating relay 31, which lights the lamp 38 and operates the buzzer- 35.

In order that the system may be properly tested, a changer and contact assembly 39 have been provided. This assembly includes pairs of contacts ill, a cam ll opening and closing said contacts, a pawl and stop 42 and the operating magnet 21. The characteristics of the magnet 21 are such that a current of about 100 milliamperes is necessary to pull up the armature 43 as compared with the normal line current of 12 milliamperes. This value of current is provided by operating section A of the key 44, which brings in the high voltage battery 45. The impulses imparted to the changer magnet 21 by means of the key 24 causes the fiber cam II to rotate in steps by means of the pawl and ratchet 22, so that three contact combinations may be obtained; namely, both pairs of contacts open as shown, the short pair closed or the long pair closed. Due to the relative width of the cam ll and the spacing of the contacts 40, both pairs of contacts are never closed at the same time.

Using the above equipment two tests can be made at the central ofiice at any time, which will give an indication of the alarm condition. For instance when the subscriber wishes to put the alarm into service he operates his key 48 which breaks the line circuit and lights the lamp 38 at the central ofllce. By prearrangement the central ofilce operator considers this as a closing signal. With the changer 38 in the position as shown and with circuit conditions correct, the needle of the galvanometer 29 will be in its center position. The operator by operating section A of the key 44 transfers the high voltage battery 45 to the line momentarily. Due to the unbalance between the windings iii and 28 of the differential relay 28, caused by the shunt 41 across the winding 28, the increased current through both windings will cause a large enough resultant magnetic field to close the relay contacts thereby providing a short path through the changer magnet 21. The changer moves the cam 4| to the point where the short inner pair of contacts 40 close. This short circuits the resistance 24 causing the line current to become 15 milliamperes and the galvanometer needle will move to the right of the center of the scale. This test is to guard against the possibility of an abnormal resistance in the line being compensated by a leakage across the circuit. Since the resistance 24 is at the end of the line circuit, such a condition would give a change on the galvanometer of difierent magnitude than normal.

For a second test the operator again transfers the battery 45 to the line momentarily, moving the cam 4| to the point where the long outer contacts 40 close, energizing relay 41 which in turn energizes relay 48. This latter step removes the short-circuit from the resistance 30 and will cause the speed of the fan II to decrease by a predetermined amount to give a change in pressure equivalent to any chosen opening in the enclosure Ill. The whole system then functions as on an alarm condition. The movement of the bellows l3 causes a rise in the oscillator current which finally results in the lighting of the lamp 34 and the operation of the buzzer-36 as heretofore described. If the above result is not achieved it indicates that the system is not capable of giving an alarm under the required conditions.

A third impulse to the line from the battery 45 rotates cam 4| to the position where both pairs of contacts 40 are open (separated by the fibre cam). In this position the differential relay 28 will still be electrically locked in the operated position since the fan does not pick up speed as quickly as the. key 44 is operated and it is necessary to break the line circuit by a partial operation of the key 44 to release the lock on the relay 28 after which the circuit will again be normal and the needle of the galvanometer 29 will again be in its central position. Movement of the key 44 to its B position then rings the alternating current bell 26 and informs the subscriber by a prearranged code signal of the condition of the alarm. Although the high current impulse for operating changer assembly 59 operates differential relay 28 on each impulse the momentary break in the line circuit caused by restoring key 44 breaks the lock on relay 28 allowing the circuitconditions as controlled by changer contacts to be shown on galvanometer 29.

Automatic supervision is afforded by the relays 22 and 23. If for any reason the plate circuit of the oscillator l should become detuned, the wiring or circuit elements become defective, or the power supply fail, one of these relays will operate, showing an alarm condition at the central office.

Loss of sensitivity will be accompanied by a decrease in the oscillator plate current, caused by the detuning of the oscillator plate circuit, which may be caused by a change in pressure which increases the distance between the bellows 3 and the fixed plate I8. The release current value of underload relay 23 affords a means for predetermining the amount of sensitivity which will give an alarm condition.

A bimetallic plate 49 near the plates of condenser |6 compensates for expansion and contraction in the plate circuit elements caused by temperature changes. Since the piezoelectric quartz crystal H in the grid circuit has an extremely low temperature coeflicient, the temperature effect on the whole system is negligible.

The bellows housing IS, with its adjustable vent 50, provides an adjustable damping chamber for the bellows. Its purpose is to give a slight delay to the bellows action so that drafts causing transient changes in the fan speed will not register as alarm signals. This adjustment is obtained by choice of length of tubing l2.

A switch 5| is provided to open the local relay battery circuit. Thus, if the changer mechanism should be left in the condition where the longouter contacts 40 are closed, the battery 52 will nbt become exhausted.

A switch 53 is provided to shut off the fan The operation of the above two switches is part of the subscribers opening and closing routine.

What is claimed is:

1. An alarm system comprising a central oilice, a subscribers station, an enclosure at said subscribers station, means including a continuously operated fan for maintaining the air in said enclosure at a pressure different from that on the outside thereof, means connected to said enclosure responsive to changes in air pressure on the interior thereof, means controlled by said second means to transmit an alarm s gnal to the central oflice upon sufficient change in pressure in said enclosure, and means, including means at the subscribers station and controlling means at said central oflice. for testing said system by varying the speed of the fan.

2. System according to claim 1, characterized in this that the testing means includes a relay at the subscribers station, a circuit leading to the central oflice and control ed therefrom, a relay in saidcircuit provided with an armature which normally short-circuits a resistance in said circuit in series with the fan motor, this relay in attracting its armature reducing the fan speed to simulate an alarm condition at the subscriber's station and thereby cause the alarm equip-- ment thereat to function.

3. An alarm system comprising a subscribers station, anenclosure at said subscribers station, means including a continuously operated fan for maintaining the air in said enclosure at a pressure different from that on the outside thereof, means connected to said enclosure responsive to changes in air pressure on the interior thereof, means controlled by said second means to transmit an alarm signal upon sufiicient change in pressure in said enclosure, and means for testing said system including means at the subscribers station for changing the normal pressure maintained in said enclosure by said fan.

4. An alarm system comprising a central station, a subscribers station, an enclosure at said subscribers station, means including a continuously operated fan for maintaining the air-in said enclosure at a pressure difi'erent from that on the outside thereof, means connected to said enclosure responsive to changes in air pressure on the interior thereof, means controlled by said second means to transmit an alarm signal to the central station upon suflicient change in pressure in said enclosure, means at said subscribers station for changing the normal pressure maintained in said enclosure by said fan, and means at the central station to operate said last-mentioned means.

LELAND H. CHASE. CLIFFORD L. LOUDON. 

